‘Seeing’ the other side of our galaxy

Astronomers have successfully traced a spiral arm on the far side of our Galaxy, an accomplishment that provides new insights into the structure of the Milky Way. Efforts to observe the far side of our Galaxy have been hampered by the vast distance and interstellar dust that blocks optical light from those regions. Here, Alberto Sanna and colleagues used radio interferometry with the Very Long Baseline Array to trace the motions of methanol and water molecules associated with a high-mass star-forming region on the far side of the Milky Way. Using the data, they were able to locate the Scutum-Centaurus spiral arm as it passes around the far side of the Galaxy and trace the arm through almost a complete rotation. The authors note that their data suggest that the pitch angle of the spiral arm (a measure of how tight the spiral is) may vary along its length. Their observations provide a record-breaking use of parallax, the apparent motion of distant objects as the Earth orbits the Sun, to measure the distance of stars. They also verify a new method of inferring distances on the far side of our Galaxy.